In many commercial situations, industrial gases, such as oxygen, nitrogen, argon and hydrogen, are supplied to a use point in a continuous manner, such as from a pipeline or from a production plant located at the site of the use point. In such continuous use situations, it is imperative, in the event the flow of the industrial gas to the use point is interrupted, that a back-up system provide gas to the use point until the primary gas supply system can be put back on line.
An industrial gas back up system typically comprises a high pressure storage tank, which holds a supply of cryogenic liquid, and a vaporizer which vaporizes the cryogenic liquid. The resulting industrial gas is then passed on to the use point. The high pressure storage tank contains a small amount of product. If the primary gas supply interruption is of extended duration, the high pressure tank requires replenishment. In such a case, cryogenic liquid is passed from a large low pressure cryogenic liquid reservoir to the high pressure storage tank using a cryogenic pump.
Before the cryogenic pump can pass cryogenic liquid from the large low pressure storage tank to the high pressure storage tank, it must be cooled down to the appropriate cryogenic temperature to ensure that the liquid does not vaporize before it reaches the pump. A cryogenic pump will not operate with vapor or even a mixture of liquid and vapor.
There are two methods used by the industrial gas industry to cool cryogenic pumps to the appropriate temperature to pump cryogenic liquid. The most widely used method is the time-based method wherein the pump is cooled for a fixed period of time prior to starting. This method is simple but can be wasteful because the proper cooling time will vary for each particular instance and thus the cooling period must be set conservatively to ensure complete cooldown under all circumstances. Moreover, this method is also disadvantageous because it increases the chances that the pump is allowed to cool too long. If the liquid in the pump is held too long, it can begin to vaporize via ambient heat leak, thus preventing pump operation. Another method is the vapor pressure method wherein a pressurized bulb of the cryogenic fluid is installed in the suction line to the pump to indicate when the proper fluid condition for pumping has been attained. This method has been unreliable because the pressurized bulb is prone to leakage over time and, moreover, this method has an inherent lag time in response because the corresponding output pressure signal requires the fluid within the bulb to condense against the outside temperature.
Accordingly it is an object of this invention to provide an improved cryogenic pump system which can undergo cooldown effectively thereby allowing reliable pump startup.